Re: fcntl(fd,F_SETSIZE,?)
Re: fcntl(fd,F_SETSIZE,?)
- Subject: Re: fcntl(fd,F_SETSIZE,?)
- From: James Bucanek <email@hidden>
- Date: Fri, 12 Oct 2012 00:05:16 -0700
- Authentication-results: symauth.service.identifier
Paul Nelson <mailto:email@hidden> wrote (Thursday, October
11, 2012 7:16 PM -0500):
I was not sure what you want to accomplish with a symlink from your original
post.
That was just coincidence. My earlier questions about symlink,
and my question about fcnt(F_SETSIZE) have nothing to do with
each other. :/
You seem interested in the inner workings of all this so here goes.
I am indeed, and I really appreciate the time you've taken to
explain it.
If you aren't, this stuff is really boring.
I live for boring.
First, you need to get the xnu sources so you can look in the kernel code.
That, right there, is a huge help. I was Googling
opensource.apple.com and wasn't finding anything I was looking for.
This is where all fcntl stuff is implemented.
I just downloaded xnu-2050.18.24 and have found the fcntl
implementation, which I trust will reveal the secrets I was
looking for.
You will need to realize that
each file system handles their implementation (or doesn't) in their own way.
Understood.
Unfortunately, the code is very hard to follow.
That's an understatement. :)
Anyway, I really value the insight into how symlinks work. I'm
going to dig into that code because I have other questions
(about deleting ACLs and whatnot) that I think are hidden there too.
My question about the F_SETSIZE fcntl() command was really about
trying to reclaim the ability to extend (physically allocate) a
file *without* filling the file with zeros. I have a couple of
performance issues where I want to pre-allocate several large
files--so I know that there's ample free space to accomodate
them--before they get populated with data. The problem is, the
standard ftruncate() function fills the files with zeros, so
they get written twice. On slower media this is a huge waste of
time (several minutes in some cases).
According to F_SETSIZE's documentation, it will permit the file
size to be extended without filling it with zeros. But the
details of the command's argument is not documented. I was
hoping a peek at the fcntl() source code would reveal that, and
now I've got it.
Thanks again,
--
James Bucanek
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